1. Field of the Invention
The present invention concerns a method of dynamically assigning subgroups of spreading sequences of a set of spreading sequences to mobile terminals of a Multi-Carrier transmission network, such as a Multi-Carrier Code Division Multiple Access transmission network, better known by the name MC-CDMA network.
2. Discussion of the Background
MC-CDMA has been receiving widespread interest for wireless broadband multimedia applications. MC-CDMA combines OFDM (Orthogonal Frequency Division Multiplex) modulation and the CDMA multiple access technique. This multiple access technique was proposed for the first time by N. Yee et al. in the article entitled “Multicarrier CDMA in indoor wireless radio networks” which appeared in Proceedings of PIMRC'93, Vol. 1, pages 109-113, 1993. The developments of this technique were reviewed by S. Hara et al. in the article entitled “Overview of Multicarrier CDMA” published in IEEE Communication Magazine, pages 126-133, December 1997.
It is known that the propagation channel can be obstructed by houses and other obstacles located between the transmitter and the receiver. The transmitted signal is then propagated on multiple paths, each path being delayed and attenuated differently. It should be understood that the propagation channel then acts as a filter whose transfer function varies with time.
The ability of MC-CDMA transmission networks to provide orthogonality between the signals of the different mobile terminals in the network (and therefore to prevent any interference between these signals) depends on the intercorrelation properties of the spreading sequences which are assigned to the mobile terminals of the network.
Typically, in the case of transmissions on a mobile radio channel from a set of mobile terminals to a base station, the signals transmitted by mobile terminals are received synchronously at the base station. The radio channel between each mobile terminal and the base station cannot be considered as an uniform channel and the same between each mobile terminal and the base station. Under these conditions, the base station has to simultaneously estimate a plurality of radio channels on each sub carrier composing the OFDM multiplex.
In their paper “An asynchronous spread spectrum multi-carrier multiple access system” Proceeding of IEEE GLOBECOM 1999, vol 1, pages 314 to 319, December 1999, S Kaiser and W. A Krzymien introduce an additional Frequency Division Multiple Access component between mobile terminals and the base station, so that the base station has to estimate only one channel for each sub-carrier. That spread spectrum multi-carrier multiple access system losses some spreading properties, which may require more stringent frequency planning to cope with cellular environment.